1. Field of the Invention
This invention relates to systems for discovering semi-conducting materials, and more particularly, to methods, materials, and devices for making and screening combinatorial libraries to identify thermoelectric materials.
2. Discussion
In its simplest form, a thermoelectric device comprises a thermoelectric material—usually a semiconductor—sandwiched between a pair of contacts. When an electrical potential is applied between the pair of contacts, heat flows from one contact to the other through the thermoelectric material. This phenomenon, which is called the Peltier effect, occurs whenever direct current flows through a junction between two dissimilar materials. Similarly, when a temperature difference is applied between the pair of contacts, an electrical potential develops which varies continuously from one contact to the other through the thermoelectric material. This latter phenomenon is called the Seebeck effect. Its size depends on the magnitude of the temperature difference, and like the Peltier effect, on the properties of the thermoelectric materials.
Thermoelectric devices exploit the Seebeck effect and the Peltier effect to generate power and to pump heat and they exhibit certain advantages over conventional compressor-based systems. For example, engineers employ thermoelectric devices to cool small volumes, such as portable food and beverage containers, medical devices, and integrated circuits, which would be-impractical to cool with bulky conventional refrigeration systems. Furthermore, thermoelectric heat pumps offer greater flexibility than compressor-based refrigeration systems since thermoelectric devices can heat, as well as cool, by simply reversing the direction of electrical current through the device. Moreover, because thermoelectric devices have no moving parts, they generate power quietly and reliably. Despite these advantages, thermoelectric devices are not used for general purpose cooling or for power generation because they are less efficient than compressor-based systems. Indeed, the most efficient thermoelectric power generators currently operate at about 10% Carnot efficiency, whereas conventional compressor-based systems operate at about 30%, depending on the size of the system.
Since efficiency and performance of thermoelectric power generators and heat pumps depend primarily on the properties of the materials used in the device, researchers continue to search for new, better performing thermoelectric materials. But, progress has been slow. Indeed, Bi—Sb—Te alloys remain the most efficient room temperature thermoelectric materials available, though they were first used in thermoelectric devices more than thirty years ago.
The slow pace of discovery is due, in part, to the time and expense of synthesizing and testing thermoelectric materials using conventional techniques. In traditional material science, researchers synthesize a few grams of a candidate material that they test or screen to decide whether it warrants further study. For thermoelectric materials, synthesis involves a labor- and time-intensive alloying process. Since material properties often depend on synthesis conditions, the discovery process usually includes a lengthy search for optimum heating and quenching cycles. In many cases, dopants are added to control microstructure, which further increases complexity of the discovery process. Although in recent years scientists have acquired a better understanding of how material structure and carrier concentration influence thermoelectric variables such as thermoelectric power, thermal conductivity, and electrical resistivity, discovery efforts continue to rely heavily on experiment.
Combinatorial chemistry is one approach for accelerating the discovery of new thermoelectric materials. It is a powerful research strategy when used to discover materials whose properties, as with thermoelectric compositions, depend on many factors. Researchers in the pharmaceutical industry have successfully used such techniques to dramatically increase the speed of drug discovery. Material scientists have employed combinatorial methods to develop novel high temperature superconductors, magnetoresistive materials, phosphors, and catalysts. See, for example, co-pending U.S. patent application “The Combinatorial Synthesis of Novel Materials,” Ser. No. 08/327,513 (a version of which is published as WO 96/11878), and co-pending U.S. patent application “Combinatorial Synthesis and Analysis of Organometallic Compounds and Catalysts,” Ser. No. 08/898,715 (published as WO 98/03521), which are both herein incorporated by reference.
The use of combinatorial materials science should enable researchers to undertake an efficient, systematic and comprehensive search of new semi-conducting or new thermoelectric materials without many of the problems associated with traditional materials development.